WhatsApp, signal etc

My biggest red flag (aside from text that seems to be either AI or from someone who's primary language is not English) is when someone messages me and almost immediately wants to take the chat to another platform or messenger. Some are certified accounts I have friended and others are just out of the blue. What's the reason for this? What are the risks? Are there any legit people on here with practical reasons for wanting to chat on signal?
Thanks